1. Field of the Invention
The present invention relates to a tool holder, and more specifically, to a tool holder for mounting a cutting tool such as drills, end mills, etc. to a main spindle of a machine tool.
2. Description of Related Art
Conventionally, it is known in the art of tool holder that the top end portion of a holder body or the rear end portion of a cutting tool is sealed for supply of coolant to processed portions or for dust prevention.
FIG. 6 shows a conventional construction for sealing the top end portion of a holder body 1. A spacer 52 is mounted to the top end portion of the holder body 1 via a cap 50 fixed with a bolt 51, and to an opening hole portion 53 of the spacer 54, an O-ring 54 is mounted. A cutting tool 40 has a shank portion 40a inserted into a collet 10 through the opening hole portion 53, and pressed and held to the collet 10. Coolant is supplied from the arrow "b" direction through the inside of the holder body 1 and spouted from the head end of the tool 40 through a center hole (not illustrated) of the cutting tool 40. Under this configuration, the coolant has pressure in the arrow "b" direction, but sealing is achieved by the O-ring 54 and an O-ring 55 inserted between the cap 50 and the spacer 52.
On the other hand, FIG. 7 shows a conventional construction in which the rear end of the cutting tool 40 is brought into contact with a top and tapered surface 15a of an intermediate rod 15 for sealing. Coolant is supplied to a center hole 42 of the cutting tool 40 through a center hole 17 of the intermediate rod 15.
Now, the sealing construction shown in FIG. 6 has a problem in that the cutting tool 40 must have a shank with a diameter same as the inside diameter of the O-ring 54. In general, various cutting tools with varying diameters in increments of 0.1 mm are used. It would be convenient if cutting tools with varying shank diameters within the range from the inside diameter of the O-ring 54 to 1 mm larger can be used with one type of spacer 52, for example, if cutting tools with shank diameters up to 13 mm in increments of 0.1 mm can be used when the inside diameter of the O-ring 54 is 12 mm.
On the other hand, in the sealing construction shown in FIG. 7, the intermediate rod 15 travels integrally with the collet 10, in other words, when the collet 10 is retracted backward (in arrow "A" direction) to tighten the cutting tool 40, the intermediate rod 15 is also retracted in this event. Consequently, pressure-contacting force between the rear end of the cutting tool 40 and the tapered surface 15a of the intermediate rod 15 is unable to be satisfactorily secured, creating a problem of insufficient sealing effect.
A conventional tool holder has a problem in that the machining accuracy is degraded by vibration in the cutting tool generated during high-speed rotation because the axial center of the cap 50 does not coincide precisely with the axial center of the holder body 1, thereby losing the balance. The cap 50 is fixed to the holder body 1 with bolts 51 using a tightening tool such as a wrench, etc., but it is difficult to fix completely coaxially to the holder body 1. In addition, because the cutting tool is rotated as high as 14,000 rpm in recent years, occurrence of vibration becomes conspicuous even if the axial misalignment of the cap 50 is negligibly small.